The invention concerns switched capacitor technology, specifically variable value switched capacitor circuits and switched capacitor filters, such as finite impulse response filters (FIR-filter), with variable value switched capacitor circuits.
Switched capacitor circuits are used for their suitability to be placed on a single semiconductor chip thereby replacing filters that have been conventionally constructed using discrete resistors, capacitors and inductors. In switched capacitor circuits, resistors and inductors are replaced by switches and capacitors.
Finite impulse response filters (FIR-filters) can be expressed as a series, for example such as             y      ⁢              xe2x80x83            ⁢              (        k        )              =                  ∑                  n          =          0                k            ⁢              xe2x80x83            ⁢              g        ⁢                  xe2x80x83                ⁢                  (          n          )                *        x        ⁢                  xe2x80x83                ⁢                  (          n          )                      ,
where g(n) is the n:th weight factor/coefficient for the n:th input value x(n). Expanded it will end up to be y(k)=g(0)*x(0)+g(1)*x(1)+g(2)*x(2)+. . . +g(kxe2x88x921)*x(kxe2x88x921)+g(k)*x(k). The series needs k+1, possibly different, weights/coefficients if it has k terms and an initial value. An implementation of such an equation would not be practical. If a simple and flexible solution to this is desired then an implementation of the recursive equation y(k)=y(kxe2x88x921)+g(k)*x(k) is preferably made. However, an implementation according to the recursive equation requires dynamically adjustable coefficients. In an implementation utilizing switched capacitor technology the value of the coefficient can be varied by, for example, dividing the sampling capacitor into a suitable number of equal sized part-capacitors, for example 256 part-capacitors. The number of part-capacitors used will depend on the desired resolution of the coefficient, a 12 bit resolution coefficient, for example, needs 4096 capacitors. During each sampling all of the sampling capacitors are charged to the input signal value. Thereafter, the input is disconnected and a number of the capacitors representing the desired filter coefficient, are connected to an output circuit, in this case an integrator. An example of such a circuit is described in the U.S. Pat. No. 5,220,286. This method of obtaining a variable coefficient circuit can suitably be named the capacitance division method. A disadvantage can be considered to be the large number of capacitances and corresponding switches that are necessary if a high resolution is desired.
An object of the invention is to define a variable value switched capacitor circuit requiring a drastically reduced number of capacitors and switches in relation to a circuit according to the capacitance division method.
Another object of the invention is to define a method of controlling a switched capacitor circuit to thereby obtain a variable value switched capacitor with a drastically reduced number of capacitors and switches in relation to a circuit according to the capacitance division method.
The aforementioned objects are achieved according to the invention by a switched-capacitor circuit comprising a plurality of sequentially-operated switched-capacitors. A first capacitor is arranged to be coupled and thereafter uncoupled to an input signal during a first time period. Thereby the first capacitor is charged to an input value in dependence on the input signal. A second capacitor is arranged to be coupled and thereafter uncoupled to the first capacitor during a second time period for charge distribution between the second and the first capacitor. Pairwise charge distribution is possibly done with further sequentially-operated switched capacitors. A final capacitor is arranged to be coupled and thereafter uncoupled to a previous capacitor during the final time period for charge distribution between the final and the previous capacitor. The switched-capacitor circuit further comprises means for selectively coupling the first and the second sequentially-operated switched-capacitors to an output circuit thereby enabling a desired weighting of the input signal to be attained.
The aforementioned objects are also achieved according to the invention by a switched-capacitor circuit comprising a plurality of sequentially-operated switched-capacitors. The capacitors are sequentially switched during a plurality of time periods. The circuit comprises at least a first, a second and a final sequentially-operated switched-capacitor. The first switched-capacitor is arranged to be coupled and thereafter uncoupled to an input signal during a first time period. The first switched-capacitor is thus charged to an input value in dependence on the input signal. The second switched-capacitor is arranged to be coupled and thereafter uncoupled to the first switched-capacitor during a second time period for charge distribution between the second and the first switched-capacitor. The charge is thus divided between the first and the second switched-capacitor. The second switched-capacitor becoming a previous switched-capacitor during the time period following the time period the second switched-capacitor received charge. The final switched-capacitor is arranged to be coupled and thereafter uncoupled to the previous switched-capacitor, in the minimum case this is the second switched-capacitor, during the final time period for charge distribution between the final and the previous switched-capacitor. The charge is thus divided between the final and the previous switched-capacitor. The switched-capacitor circuit further comprises means for selectively coupling the first and the second switched-capacitors to an output circuit thereby enabling a desired weighting of the input signal to be attained.
Advantageously, to achieve a higher weight factor resolution, the circuit further comprises a predetermined number of additional sequentially-operated switched-capacitors. The additional capacitors are arranged between the second switched-capacitor and the final switched-capacitor. Each additional sequentially-operated switched-capacitor is arranged to be coupled and thereafter uncoupled to a previous switched-capacitor during a time period for charge distribution between an additional switched-capacitor in question and the previous switched-capacitor. The charge is thus divided between the additional switched-capacitor in question and the previous switched-capacitor. The additional switched-capacitor in question becoming the previous switched-capacitor during the time period following the time period the additonal switched-capacitor in question received charge. The switched-capacitor circuit further comprises means for selectively coupling each additional sequentially-operated switched-capacitor to an output circuit.
Some embodiments of the switched capacitor circuit further comprises a first parallel-operated capacitor. The first parallel-operated capacitor is arranged to be coupled and thereafter be uncoupled to the input signal during the first time period. The first parallel-operated capacitor is not involved in any charge sharing with the sequentially-operated switched-capacitors. The circuit further comprises means for selectively coupling the first parallel-operated capacitor to an output circuit.
Advantageously each means for selectively coupling a sequentially-operated capacitor to an output circuit is for each capacitor in question selectively activated during a time period which is after a time period when the capacitor in question distributed its charge to a further capacitor.
Preferably each capacitor which is not coupled directly to the input signal is shorted at the latest during a time period before a time period in which a capacitor in question receives a charge distribution from a previous capacitor.
In certain embodiments the circuit is doubled into two identical branches, the operation of the branches being time shifted in relation to each other, thereby doubling the throughput rate. In some version the circuit is quadrupled into four identical branches, the operation of the branches being time shifted in relation to each other, thereby quadrupling the throughput rate. In some versions the circuit is arranged to be coupled to one output for every four sequentially-operated switched-capacitors that can be selectively coupled to an output circuit.
Preferably all the capacitors all have the same capacitance thereby enabling a selectable linear binary weighting of the input signal.
Advantageously the circuit comprises further more parallel-operated input capacitors. The circuit also further advantageously comprises more parallel-operated least significant bit switched-capacitors. In some embodiments the final switched-capacitor consists of a plurality of parallel-operated least significant bit switched-capacitors.
To enable weighted summations some embodiments of the circuit further comprises output means in the form of an integrator, thereby providing a computing-element for weighted summations. The circuit is in some embodiments a filter which filters the input signal. The circuit can then for example be a finite impulse response filter working according to a series development. The circuit can also comprise two output circuits, one output giving a quadrature value and the other output giving an in-phase value.
Preferably the circuit is completely differential to thereby handle both positive and negative weight factors. Other important advantages with these embodiments of the invention is that the circuit will be immune to supply noise and achieve a high common mode rejection ratio. In some embodiments the circuit comprises a reversing switch to reverse a polarity of the input signal to thereby enable both positive and negative weight factors.
The different additional enhancements of the invention can be combined in any desired manner as long as not any conflicting features are combined. The aforementioned objects are also achieved according to the invention by a switched-capacitor circuit which comprises a plurality of sequentially-operated switched-capacitors. The first of which is coupled to an input signal during a first time period thereby charging the first switched-capacitor to an input value in dependence on the input signal. Thereafter the charge on the first switched capacitor is sequentially charge distributed during further time periods. Each sequential charge distribution is between pairs of sequentially-operated switched-capacitors. First the charge is divided between a first pair of sequentially-operated switched capacitors consisting of the first sequentially-operated switched-capacitor and a second sequentially-operated switched-capacitor. Then during a next time period the charge is divided between a second pair of sequentially-operated switched-capacitors consisting of the second sequentially-operated switched-capacitor and a third sequentially-operated switched-capacitor. This is repeated between pairs of sequentially-operated switched-capacitors until a last sequentially-operated switched-capacitor belongs to a pair of switched-capacitors performing charge division. The switched-capacitor circuit further comprises means for selectively coupling the switched-capacitors to an output circuit thereby enabling a desired weighting of the input signal to be attained.
The aforementioned objects are also achieved according to the invention by a switched-capacitor circuit comprising a plurality of sequentially-operated switched-capacitors. An input sequentially-operated switched-capacitor is arranged to be coupled and thereafter uncoupled to an input signal during a first time period. The input switched-capacitor is thus charged to an input value in dependence on the input signal. For a following time period the input switched-capacitor becomes a previously charged switched-capacitor. A predetermined number of further sequentially-operated switched-capacitors are arranged to sequentially perform a number of steps. Of which further sequentially-operated switched-capacitors, a discharged current switched-capacitor is arranged to be coupled and thereafter uncoupled to the previously charged switched-capacitor during a time period for charge distribution between the current and the previously charged switched-capacitor. The charge is thus divided between them. Thereafter the current switched-capacitor becomes the previously charged switched-capacitor and the process-steps are repeated until all further sequentially-operated switched-capacitors have become the previously charged switched-capacitor. A final switched-capacitor is arranged to be coupled and thereafter uncoupled to the previously charged switched-capacitor during the final time period for charge distribution between the final and the previously charged switched-capacitor. The charge is thus divided between the final and the previously charged switched-capacitor. Then by further coupling by coupling means selected switched-capacitors to an output circuit a desired weighting of the input signal can be attained.
The aforementioned objects are also achieved according to the invention by a method of weighting an input signal by means of a switched-capacitor circuit comprising a plurality of switched-capacitors. According to the method the switched capacitors are sequentially switched during a plurality of time periods. The method comprises a number of steps. In a first step a first switched-capacitor is charged to an input value in dependence on the input signal by coupling and thereafter uncoupling the first switched-capacitor to an input signal during a first time period. Thereafter in a second step the charge on the first switched-capacitor is divided between the first switched-capacitor and a second switched-capacitor by coupling and thereafter uncoupling the second switched-capacitor to the first switched-capacitor during a second time period for enabling charge distribution between the second and the first switched-capacitor, the second switched-capacitor becoming a previous switched-capacitor during the third time period. Thereafter in a final charge division step the charge on the previous switched capacitor is divided between the previous switched-capacitor and a final switched-capacitor by coupling and thereafter uncoupling the final switched capacitor to the previous switched-capacitor during a final time period for enabling charge distribution between the final and the previous switched-capacitor. The first switched-capacitor and the second switched-capacitor are also selectively coupled to an output circuit to thereby enable a desired weighting of the input signal to be attained.
Advantageously to further extend the resolution of the weigthing factor the method further comprises a predetermined number of addional steps. The additional steps are preferably performed between the second time period and the final time period and performed on one or more additional switched-capacitors. The additional steps comprises a step that is performed repeatedly on one additonal switched capacitor at a time. The step comprises dividing the charge on the previous switched-capacitor between the previous switched-capacitor and an additional switched-capacitor in question by coupling and thereafter uncoupling the additional switched-capacitor in question to the previous capacitor during a time period for charge distribution between the additional switched-capacitor in question and the previous switched-capacitor. The additional switched-capacitor in question thereafter becomes the previous switched-capacitor for a time period directly following the time period for charge distribution between the additional switched-capacitor in question and the previous switched-capacitor. The method also comprises the additional step or steps of selectively coupling each additional switched-capacitor to the output circuit.
In some versions of the method each step of selectively coupling a switched-capacitor to an output circuit is for each switched-capacitor in question selectively activated during a time period which is after a time period when the switched-capacitor in question distributed its charge to a further capacitor.
In other versions the method further comprises the step of coupling and thereafter uncoupling a first parallel-operated switched-capacitor to the input signal during the first time period, and selectively coupling the first parallel-operated capacitor to an output circuit.
In still other versions for each switched-capacitor which is not coupled directly to the input signal the method comprises the step of shorting a switched-capacitor in question at the latest during a time period before a time period in which the switched-capacitor in question receives a charge distribution from a previous capacitor.
The different additional enhancements of the method according to the invention can be combined in any desired manner as long as not any conflicting enhancements are combined.
By providing a variable value switched-capacitor circuit a plurality of advantages over prior art systems are obtained. A primary purpose of the invention is to reduce the number of capacitors and switches needed for a specific resolution. For a 12 bit binary coefficient/weight factor, in a basic embodiment of the invention with charge sharing, 13 equal sized capacitors and approximately 40 switches are needed. These numbers should be compared with the 4096 equal sized capacitors and more than 8000 switches that are needed with the capacitance division method. According to the invention this reduction is achieved primarily by sequential charge sharing between pairs of capacitors. The use of a low number of a equal sized capacitors enables a higher accuracy to be achieved as it is easier to manufacture a limited number of equal sized capacitors than manufacturing a large number of equal sized capacitors or a scale of different sized capacitors. Speed improvements are easily attained by adding parallel processing paths.
Other advantages of this invention will become apparent from the detailed description.